The service water system may, for example, be a heating system. Such a heating system is filled or re-filled from a drinking water system, the drinking water supply. It has to be ensured, at all events, that water does not flow from the service water system back into the drinking water system, for example in the case of pressure drop in the drinking water system. To this end, “backflow preventers” are provided. These backflow preventers are spring loaded check valves, which, under the influence of the drinking water pressure open only in the direction from the drinking water system towards the service water system. Also this, however, is not regarded as sufficient for continuous operation. Rather is a physical separation between the drinking water system and the service water system prescribed, for example by filling or re-filling through a hose which is removed, after the filling or re-filling process has been completed. This ensures that no service water can get into the drinking water system even in the case of leaking shut-off valves or backflow preventers.
As the removal of the hose after the filling or re-filling process is troublesome and, in addition, cannot be checked, fixed installations of pipe disconnectors are known (for example EP 0,972,995 A1). These known pipe disconnectors comprise an upstream backflow preventer, i.e. a backflow preventer installed on the side of the drinking water system, and a downstream backflow preventer, i.e. a backflow preventer installed on the side of the service water system. Both backflow preventers open in the direction towards the service water system. A pressure controlled relief valve is provided between the backflow preventers. This relief valve is controlled by the drinking water pressure and opens automatically, when the drinking water pressure breaks down or drops. Thus, if the service water system is filled or re-filled and a service water pressure sufficient therefor is present, then the relief valve is closed by this pressure. Drinking water flows through the backflow preventer pushed open by the drinking water pressure and into the service water system. If the drinking water pressure drops below a predetermined level, either because a shut-off valve shuts off the drinking water system or because the drinking water pressure breaks down for one reason or another, the relief valve will open. Even if then service water flows back from the service water system through a leaking backflow preventer, this service water flowing back is drained through the outlet and, by no means, can get into the drinking water system.
In a prior art design, the relief valve has a sleeve-shaped valve closure body, which cooperates with an annular valve seat on a substantially tubular pipe disconnector casing. The valve closure body is biased by a spring in the direction towards the open position. The backflow preventers and the valve closure body are arranged coaxial within the pipe disconnector casing. The valve closure body is supported non-positively on a plate displaceably guided in the pipe disconnector casing, the loading spring of the upstream backflow preventer being, in turn, supported upstream on this plate. When sufficiently high pressure is present in the drinking water system, the valve closure body is pressed on the valve seat through displaceable plate against the action of the spring. Then the upstream backflow preventer is pushed open. The inflowing drinking water pushes the downstream backflow preventer (“Water Regulations Guide” issued by WRAS, 6.15).
In the prior art pipe disconnectors, the pressure situation is not exactly defined and cannot be verified. The movement of the valve closure body into its open position is effected only by the spring acting on the valve closure body, this spring having to keep the valve closure body in non-positive engagement with the plate, if this plate yields.
These so-called pipe disconnectors of the type CA are intended for a certain risk class of the service water. There are service water classes having a higher level of contamination and involving a correspondingly higher risk. The pipe disconnectors described above are regarded as insufficient for the separation of such service water from the drinking water. Here, the standards demand pipe disconnectors of the so-called type BA providing increased safety.
This is achieved by providing a median pressure zone between the upstream and downstream backflow preventers, the relief valve being differential pressure-controlled by the pressure difference between drinking water system and the median pressure zone. This ensures, with each hydraulic situation, that a pressure drop from the drinking water system to the median pressure zone exists. In known manner, the relief valve is controlled by a diaphragm, across which the pressure difference acts. If service water enters the median pressure zone, the pressure in the median pressure zone will rise, and the relief valve will open to maintain a constant pressure difference (“Water Regulations Guide”, issued by WRAS, 6.14)
With the pipe disconnectors of the type BA, test taps for the connection of pressure gauges are provided, by means of which the pressures of drinking water and service water and the “medium pressure” in the space between the backflow preventers can be measured.
The prior art pipe disconnectors with differential pressure-controlled relief valve are of expensive construction. Cleaning and servicing is difficult, because the individual components are not, or only with difficulties, accessible. Basically different valve assemblies are used for pipe disconnectors of the type CA and for pipe disconnectors of the type BA.
A company brochure “SYR Füllgruppe Typ 2128” of Hans Sasserath & Co. KG, describes a filling unit, which is permanently installed at a service water system such as a closed hot water heating installation and has a connector for connection of a hose. The filling unit can be connected to a drinking water system through a hose to be connected to this connector. This filling unit includes a backflow preventer, a shut-off valve and a pressure reducer.